In a lens-coupled electron microscope camera, when the image sensor is not in the microscope vacuum, it is necessary to have two optically-transmitting vacuum walls (windows), one to contain the image sensor so that it can be cooled without accumulating condensation from the atmosphere and one to act as a continuation of the vacuum wall of the microscope to maintain integrity of the electron microscope vacuum system and x-ray safety shielding while still transmitting light with minimal deflection and scattering so that an image of the electron-illuminated scintillator can be formed outside the microscope vacuum.
The presence of these two extra layers of glass, which do not in general contribute to the action of the lens, and the fact that the inner one must address the need for sufficient mass thickness to block x-rays produced in the microscope, especially at the scintillator and supporting structures, and the four extra reflective surfaces added by these two glass elements, together have one or more of the following consequences: (1) They reduce the sharpness of the image formed at the sensor. (2) They incur the need for custom optical design in the lens for the purpose of compensating those aberrations. (3) They increase the light absorption and thereby reduce the sensitivity of the system. (4) They increase the problem of scatter and ghost images due to reflections at the 4 additional optical surfaces. (5) They increase the object-to-image distance of the optical system thereby limiting the maximum collection angle possible with a given lens diameter and consequently limit the sensitivity of the lens unless the lens-element diameter is increased proportionally, with associated difficulties with the mechanical interface to the microscope. (6) Finally, the additional elements increase the cost of the system, both directly and by the cost of the measures needed to compensate their defects when it is necessary to do so.
One possible mitigation of the added-window problem is to integrate the vacuum windows into the design of the lens, making the first lens-element the microscope vacuum window and the last, the image sensor vacuum window. However, this approach leads to other problems since it mixes the mechanical constraints, forces and x-ray shielding needs of the microscope vacuum barrier with the delicate constraints of realizing the imaging optics.
When the image sensor is mounted outside the microscope vacuum in a retractable camera, it becomes difficult to provide for correct focal alignment during operation. It is necessary to provide a mechanical linkage independent of the optical chain itself which either runs in parallel to the optical chain or surrounds it. This linkage must provide consistent alignment over repeated insertions and retractions of the optical system in the presence of the forces generated by air pressure outside the microscope vacuum on the movable optical system. One approach has been to place the mechanical linkage from image sensor to scintillator to the sides of the optical system with small cross-sectional area to minimize air-vacuum barrier forces and leave the microscope vacuum window fixed.
Thus there remains a need for a method of housing a scintillator, lens and image sensor for an electron microscope which simultaneously provides high-quality and high-sensitivity optical coupling, and alignment stability during insertion and retraction.
The following references are relevant to the background of the invention and are incorporated herein by reference. U.S. Pat. No. 5,065,029, Cooled CCD Camera for an Electron Microscope; U.S. Pat. No. 5,536,941, Rotatable Wide-Angle Camera and Prism Assembly for Electron Microscopes; U.S. Pat. No. 6,061,085, Camera System for a Transmission Electron Microscope, and Japanese Unexamined Patent Application H08-264148 by Tokuichiro Hayashi, filed Mar. 23, 2005.